Principal Investigator: Hongzhe Li Program Summary Project Title: Maximizing hearing recovery from peri-synaptic damage Program Summary Ototoxicants such as aminoglycoside antibiotics, and anti-neoplastic cisplatin, cause cytoplasmic stress within the sensory hair cells and the spiral ganglion neurons, affecting synaptic functionalities and signal transmission towards the central auditory system. We hypothesize that ototoxic cochlear synaptic damage that to some extent resembles noise-induced synaptopathy, accounts for the observation that after various ototoxic insults, within particular dosing range, without effective intervention, the auditory functions deteriorate permanently, despite of minimal or no hair cell loss. Thus, in the present project, we will investigate the similarity and discrepancy of synaptic damage due to noise or aminoglycosides and decipher the cause of synaptopathy at cellular and molecular levels. The proposed project is designed to investigate the aminoglycoside treatment conditions, which result in classic synaptopathy, and to search for optimal therapeutic temporal windows and candidate agents to intervene with degeneration process. In this manner, study findings will permit maximal hearing recovery after either noise over-stimulation or exposure to ototoxic insults. The specific aims of this project are to: First, determine the optimal aminoglycoside dosage that produces maximal ototoxic synaptopathy without functional hair cell damage in CBA/CaJ mice. An established 14-day gentamicin protocol will be used, with various dosing strategy. Electrophysiological and acoustical measures, including auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs), will be used to assess post-synaptic (ABR) and pre-synaptic (DPOAE) outer hair cell function, respectively. Second, using the optimal gentamicin dosing, we will characterize the dynamic synaptic modification in ototoxic synaptopathy. Here, we will conduct morphological investigation to visualize the synaptic variation and the survival of spiral ganglion neurons at multiple time points after the initiation of gentamicin treatment. Third, we will determine the effects of cochlear inflammation on synaptic damage, using genetically modified mouse models including Darc and TrpV1 knockout mice. Both strains of mice present certain degree of resistance to noise-induced hearing loss. Last, we will test potential audiologic rehabilitation strategies for synaptopathy, focusing on inflammation suppressive corticosteroids. This is the major rehabilitation component of the project, we will perform intratympanic injection of several therapeutics in the models of ototoxic and noise-induced synaptopathy. Auditory function will be assessed by ABR and DPOAE at several post-treatment time points, and synaptic element examined by immunolabeling and microscopy. Cochlear synaptopathy plays an essential role in auditory damage, likely affecting the supra- threshold auditory functions. These functions are critical for frequency selectivity and temporal processing, both important for speech understanding and listening in the noise environment. This creates an extreme adverse situation in military settings when effective interpersonal communication means life-or-death, and greatly affects the life quality of Veterans. This research proposal is ultimately to improve the effectiveness of rehabilitation from synaptic damage in the inner ear.